Biological Rhythms: Circadian rhythms - PowerPoint PPT Presentation

Biological Rhythms: Circadian rhythms . Aims To know the biological clock in control of the Circadian Rhythm To understand the difference between Endogeous Pacemakers and Exogenous Zeitgebers To be able to explain research into Circadian rhythms. Biorhythms .

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Our internal rhythms are thought to be generated by protein synthesis within the SCN. Protein is produced for a period of hours until it reaches a level that inhibits further production. Over the next few hours the protein level gradually falls, when it drops to a certain ‘threshold’ level then production of the protein re-starts. This generates an internal (endogenous) biological rhythm – in humans of between 24 ½ and 25 hours.

The basis of the circadian rhythm lies in interactions between certain proteins, creating the ‘tick’ of the biological clock; it is an ingenious negative feedback loop.

Darlington et al. (1998) first identified such proteins in the fruit fly, drospholia. In the morning, two proteins, CLOCK and CYCLE (CLK-CYC) bind together. Once joined, CLK-CYCproduce two other proteins, PERIOD and TIME (PER-TIM). PER-TIM has the effect of rendering the CLK-CYC proteins inactive, so that, as PER-TIM increases, CLK-CYC decreases and therefore PER-TIM starts to decrease too (negative feedback). This loop takes about 24 hours and, hey presto, you have the biological clock!

The actual proteins vary from animal to animal. In humans the main pairs are CLOCK-BMAL1 and PER-CRY (BMAL1 and CRY are also proteins). This protein mechanism is present in the SCN (the central oscillator), and is also present in cells throughout the body (peripheral oscillators). The presence of peripheral oscillators explains why there are different rhythms for different functions such as hormone secretion, urine production, blood circulation and so on.